4056
Langmuir 1995,11, 4056-4060
Orientation Change of Dimer-Type Porphyrin in Langmuir-Blodgett Films Caused by a Trigger Molecule Reiko Azumi" and Mutsuyoshi Matsumoto National Institute of Materials a n d Chemical Research, Higashi, Tsukuba, Ibaraki 305, Japan
Shin-ichi Kuroda Electrotechnical Laboratory, Umezono, Tsukuba, Ibaraki 305, Japan
Lionel G. King CRC Molecular Engineering a n d Technology, Homebush, New South Wales 2140, Australia
Maxwell J. Crossley School of Chemistry, The University of Sydney, New South Wales 2006, Australia Received March 22, 1995. I n Final Form: July 27, 1995@ The orientation of a conjugated dimer-typecopper(I1)porphyrin without a long alkyl chain was controlled by adding a small amount of long chain n-alkane such as n-hexatriacontane (CH3(CH2)3&H3)in the mixed LB films with cadmium icosanoate. The orientation ofthe dimer-typeporphyrin was investigated by using the electron spin resonance (ESR)spectroscopyfor the mixed LB films with the molar mixing ratio porphyrin/ cadmium icosanoateln-alkane= 1.51101~ in the range 0 5 x 5 5. Without n-alkane,the dimer-typeporphyrin had an orientation tilted with the most probable angle 0 zz 60" between the plane normal of the molecule and the film normal, while the macrocycle plane of the porphyrin was oriented more vertically (0 = 75") with respect to the film surface for the LB film with the mixing ratio x = 2.0 of n-hexatriacontane. The orientation change depended on the chain length of n-alkane. When the amount of n-alkane was kept at 2.0, n-alkanes with chain length more than 28 acted effectively as triggers whereas n-octadecane could not cause an orientational change. This indicates that there is a certain lower limit to the length of n-alkane for the molecule to act as an effective trigger. Polarized Uvlvis spectroscopic study of the mixed LB films, combined with that of the molecular Orientation in the oriented liquid crystal, indicates that the PD molecule lies as "side-on"on the film surface in the n-hexatriacontane-containing LB film. The role of the trigger molecule was such that the angle between the shorter molecular axis of PD and the film surface is controlled by the trigger.
Introduction Porphyrin derivatives have been extensively studied from the viewpoint of electron transfer associated with the photosynthetic centers and the applications to electronic devices.l Many efforts have been made to control their electric andor magnetic properties by changing chelating metals and the peripheral structures. The conjugated dimer-type porphyrin shown here (Figure 1)is a compound with interesting redox behavior~.~,~ The free-base, dicopper, and dizinc dy-ivatives of this compound show four reversible one-electron reductions to form the isoelectronic structure of the bis-n-anion radical, indicating that the two conjugated porphyrin rings are in communication. They have significantly narrowed HOMO-LUMO energy gapse3Moreover, as shown later, ESR spectra clearly indicate that two unpaired electron Abstract publishedinAdvance ACSAbstracts, October 1,1995. D.,Ed. The Porphyrins; Academic Press: New York, 1978. (2)Crossley, M.J.;Burn, P. L. J.Chem. Soc., Chem. Commun. 1987, 39. (3)(a) Crossley, M.J.; Burn, P. L. J. Chem. SOC.,Chem. Commun. 1991, 1569.(b) Burn, P. L.; Crossley, M. J.; Lay, P. A. In Electrochemistry-Current a n d Potential Applications; Royal Australian Chemical Institute, Electrochemistry Division: Kensington, 1988; p 245. @
(1)Dolphin,
0743-746319512411-4056$09.0010
1, M=2H 2, M=Cu 3. M=Zn Figure 1. Molecular structure of dimer-type porphyrin.
spins of dicopper derivatives are interacting through a Heisenberg exchange interaction on two copper nuclei in a molecule. This feature should particularly be important when different metal cations are incorporated in a molecule. Further, the molecule has a shape stretched in one direction, which may be utilized when the orientation of the molecule is to be controlled. On the basis of these results, the next step to explore the possibilities of this class of compounds is to construct molecular assemblies comprising these compounds, aiming a t controlling the orientation of the molecules. To achieve this, the Langmuir-Blodgett (LB) technique is the most promising one since nonamphiphilic molecules 0 1995 American Chemical Society
Langmuir, Vol. 11, No. 10, 1995 4057
Orientation Change of Dimer-Type Porphyrin as well a s amphiphilic ones have been successfully incorporated in LB films. In this respect, we recently found that a small amount of long-chain n-alkane such as n-hexatriacontane ( C H ~ ( C H ~ ) S ~ C when H ~ )added , in the preparation of monolayers, can drastically change the orientation of a porphyrin molecule in the LB film^.^!^ The ESR spectra showed that, in the mixed LB films with the molar mixing ratio meso-tetrakid 3,5-di-tert-butylphenyl)porphinatocopper(I1) (PM)/cadmium icosanoateln-hexatriacontane = 1.5/10/x in the range 0 9 x I5 , the major component of PM without n-hexatriacontane (3c = 0) had a n orientation tilted with the most probable angle 6 0 = 58" between the plane normal of the molecule and that of the film surface, while the macrocycle plane of the major component was oriented almost vertically (60 = 80') with respect to the film surface forx = 0.5 (one-third the number of PM molecules) of n-hexatriacontane. These results indicate that, by using a trigger molecule, which in this case is a long chain n-alkane, we can control the orientation of the functional molecules in the LB films without chemical modifications. Similar results were obtained for chemically modified PM's in the mixed LB films.6This implies that functional molecules in general, if endowed with certain characteristics, may have their orientation controlled in molecular assemblies when mixed with appropriate trigger molecules. In this paper we will demonstrate that the same type of molecule can also serve as a trigger for the copper derivative of dimer-type porphyrin PD, which has an elongated shape, in the mixed LB films.
Experimental Section Surface Pressure-Area Isotherm Measurement and Film Preparation. The dicopper derivative of dimer-type porphyrin, PD, was synthesized as described beforeS2Icosanoic acid (Eastman Kodak) and n-alkanes (CH3(CH2),-2CHa, n = 18, 24, 28, 36, 40, Wako Chemicals) were used as received. A chloroform solution of molar ratio PD/icosanoic acidln-alkane = 1.5/10/x (0 5 x 5 10) was spread onto an aqueous subphase of M of pH 6.0,containing 4.0 x lo-* M of CdClz and 5.0 x KHCO3 at 17 C. Icosanoic acid was converted to cadmium salt (C20) on this ~ u b p h a s e .All ~ the monolayer experiments were performed a t 17 "C on a Lauda Filmwaage. The monolayers were transferred onto a solid substrate by the vertical dipping method at a surface pressure of 25 mN m-l. The dipping speeds were 10 and 15 mm min-' for upward and downward strokes, respectively. A polyethyleneterephthalate sheet precoated with five monolayers of cadmium icosanoate was used as a substrate for electron spin resonance (ESR)measurements,8 and a quartz was plate hydrophobized with l,l,1,3,3,3-hexamethyldisilazane used for UV/vis spectroscopic studies. The transfer ratio near unity was obtained for all the monolayers except the one with the mixing value x = 10.0, which could not be transferred to the substrate under the present conditions. Study of the Molecular Orientation in Oriented Liquid The quartz plate was wiped in one direction several Cry~tal.~ times with a cotton tissue. Adrop of nematic liquid crystal (Merck E7) containing a small amount of PD molecule was mounted onto the wiped plate and sandwiched with another piece of quartz plate. The sample was heated on a hot plate and cooled slowly. The UV/vis absorption measurements were done as described later. Electron Spin Resonance Measurements and the Simulation of the Spectra. The X-band ESR spectra of the mixed O
(4)Azumi, R.; Matsumoto, M.; Kawabata, Y.; Kuroda, S.; Sugi, M.; King, L. G.; Crossley, M. J. J . Am. Chem. SOC.1992,114,10662. (5) Azumi, R.; Matsumoto, M.; Kawabata, Y.; Kuroda, S.; Sugi, M.; King, L.G.; Crossley, M. J. J. Phys. Chem. 1993,97,12862. (6) Azumi, R.; Tanaka, M.; Matsumoto, M.; Kuroda, S.;Sugi,M.; Le, T. H.; Crossley, M. J. Thin Solid Films 1994,242, 300. (7) Lieser, G.; Tieke, B.; Wegner, G. Thin Solid Films 1980,68,17. (8)Kuroda, S.; Sugi, M.; Iizima, S. Thin Solid Films 1983,99,21. (9) Michl, J.; Thulstrup, E. W. Spectroscopy with Polarized Light; VCH Publishers, Inc: New York, 1986.
60
E
z
5
40
a3
3
g
3 0
20
5 0 0
0.2
0.4
0.6
0.8
1 .o
Area per Czo I n W
Figure 2. Surface pressure-area isotherms of the mixed monolayers with PDkadmium icosanoateln-hexatriacontane = 1.5/10/x: (a)x = 0; (b) x = 0.5; (c) x = 2.0; (d) x = 5.0; (e)x = 10.0. LB films were obtained with a Varian E-4 ESR spectrometer at room temperature. A 40-layer film was placed in the ESR cavity with the film surface perpendicular or parallel to the external magnetic field. A preliminary simulation of the spectra was performed according to the previously described procedure,jJOJ1assuming the angular distribution function for the majority ofPD molecules, given as follows,
p(e)= exp{-sin2 (e - e0)/2 sin2 s,> + exp{-sin2 ( 8
+ 8,)/2 sin2 do} (1)
where 0 represents the angle between the surface normal of the film and the plane normal of the porphyrin ring, 00 is the angle for the most probable orientation, and 60 is the distribution parameter. The details have been described p r e v i ~ u s l y . ~ Polarized UVNis Measurements. The UV/vis absorption spectra were obtained for four-layered LB films and the oriented liquid crystal samples with a JASCO HSSP-1 UV/vis spectrophotometer. To investigate the in-plane anisotropy, the samples were mounted at a n incident angle 0", using polarized light with the electric vector parallel or perpendicular to the dipping direction for the LB films and the wiped direction for the oriented liquid crystal. For the out-of-planeanisotropy, the LB films were mounted a t an incident angle 45" using p- and s-polarized light with the electric vectors parallel and perpendicular to the incident plane. No in-plane anisotropy was observed for any of the LB films discussed in this paper.
Results and Discussion Surface Pressure-Area Isotherms of the Mixed LB Films. Monolayer measurements give a n average value of the area occupied by PD at the air-water interface. Figure 2 shows the surface pressure-area isotherms of the mixed monolayers with and without n-hexatriacontane (CHdCHd34CHd. The limiting area per PD molecule is estimated to be ca. 2.6 nm2, assuming that C20 occupies 0.20 nm2, in the mixed monolayer without n-hexatriacontane. This value is a little smaller than the side-on molecular size of PD (1.3nm x 3.2 nm = 4.2 nm2)and a little larger than the stand-on one of PD (1.5 nm x 1.3 nm = 2.0 nm2)roughly estimated from the space-filling model.3 The area per C20 does not change so much with the mixing value x up (10)(a) Kuroda, S.; Ikegami, K.; Sugi, M.; Iizima, S. Solid State Commun. 1986,58,493. (b) Kuroda, S.; Ikegami, K.; Sugi, M. Mol. Cryst. Liq. Cryst. 1990,190,111. (c) Kuroda, S.; Ikegami, K.; Tabe, Y.; Saito, K.; Saito, M.; Sugi, M. Phys. Rev. 1991,B43,2531. (11)Kuroda, S. Colloids Surf A 1993,72,127.
Azumi et al.
4058 Langmuir, Vol. 11, No. 10, 1995 to 5.0 of n-hexatriacontane. This suggests two mechanisms: the apparent area is nearly constant since (1)a probable increase in the area due to n-hexatriacontane is compensated by the decrease in the area due to the different molecular arrangement of PD for 0 < x 5 5.0 compared with the one forx = 0, and (2)n-hexatriacontane fills the vacant space present in the LB films (this point will be discussed further in the next section). The increase in the area a t x = 10 suggests that such mechanisms do not exist for the accommodation of n-hexatriacontane in the films with 3c > 5. Similar features, nearly constant area followed by a n increase in area with increasing x, were also observed for the mixed monolayer of PM, the monomer-type p ~ r p h y r i n .The ~ maximum value of x for the constant area in the PD case, however, is 5.0, significantly larger than the value of 1.0 for the PM case, which may be ascribed a t least in part to the larger size of PD compared with PM. The mixed monolayer with other n-alkanes (CH3(CH2In-2CH3: n = 18,24,28,40)withx = 2.0 gave similar surface pressure-area isotherms and occupied areas. Electron Spin Resonance Spectra of the Mixed LB Films. The ESR spectroscopy is a powerful tool for elucidating the structure of the LB films, especially for investigating the orientation and angular distribution of paramagnetic species in the LB films.8s10-14One of the advantages of this technique is that we can obtain the distribution function of the orientation of the probe molecule compared with other spectroscopic techniques such as polarized Uvlvis and IR spectroscopies which give only the average tilt angles of specifictransition moments of the probe molecule. The hyperfine interaction of the unpaired 3d electron of copper(I1) ion with its own nucleus (Z.= 3/2) causes the signal to split into four lines. For the square planar copper(11) complexes, the g value and the hyperfine coupling constant A are expressed as follows
g ( q l 2= g,,2cos2 q + g, 2 sin2 q
A(q)' = A,,2cos2 q
+ A:
sin2 q
(2)
(3)
where vj is the angle between the external magnetic field direction and the main symmetry axis ofthe paramagnet, glland g, are the g values when the external magnetic field is parallel and perpendicular to the main symmetry axis, respectively,AllandAl are the corresponding coupling constants. For usual copper porphyrins, these values are cm-l, g, 2.05, AI obtained as follows: AI % 3 x 2x cm-l,gIl% 2.18.15 Such anisotropic features have been used to determine the orientation of planar copper complexes with respect to the external magnetic field. For the moleculewhich has two unpaired electrons, each interacting with a nuclear spinlwith equivalent hyperfine coupling constant A, it is well known that the spectra show drastic dependence on the magnitude of Heisenberg exchange interaction J between two unpaired electron (12) (a) Messier, J.;Marc, G. J. J . Phys. (Fr.) 1971,32,799. (b) Suga,
K.;Iwamoto, Y.; Fujihira, M. Thin Solid Films 1994,234,330. (13)(a) Cook, M. J.;Dunn,A. J.; Gold,A. A,; Thomson, A. J.;Daniel, M. F. J. Chem. SOC.,Dalton Trans. 1988,1583.(b) Cook, M. J.;Daniel, M. F.; Dunn, A. J.; Gold, A. A,; Thomson, A. J. J. Chem. SOC.,Chem. Commun. 1986,863. (14)(a) Pace, M. D.; Barger, W. R.; Snow, A.W. Langmuir 1989,5, 973.(blvandevyver, M.; Barraud,A.; Ruaudel-Teixier, A,;Maillard, P.; Gianotti, C. J. Colloid Interface Sci. 1982,85,571. (15)Lin, W. C. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1979;Vol. 4,p 355.
a i
,w
I'
' ,
. - -q ,
=---
I
HI/
.. . . .... ..
,..
200G I
_.............-.. --..
' ,
I
1 I
I " ,
Figure3. First derivative ESR spectra ofthe LB film (40 layers on both sides)with the mixing ratio PDICBOIn-hexatriacontane = 1.5/10/0(a)and 1.5/10/2.0(b): (upper)the external magnetic field perpendicular (HL) and (lower) parallel (&I) to the film surface. Dashed lines represent simulated spectra. Stick lines show the septet Cu hyperfinestructure due to theAllcomponent. spins, relative to that ofthe coupling constantA.16 When the exchange interaction is small ( J % 01, the ESR signal is similar to the one for the monomer. When two unpaired electrons are separated and the exchange interaction is large (in the case of J >>A),on the other hand, the signal exhibits the features of a spin interacting with two nuclear spins, the line spacing being a half of the coupling constant A. Thus, in the case of Cu2+ dimer with Z = 312, the hyperfine splittingbecomes a 1:2:3:4:3:2:1septet with the whole signal range unchanged. Figure 3a shows the spectra ofthe mixed LB film without n-alkane, with the external magnetic field perpendicular (H,) and parallel (HI11 to the film surface. Each signal shows a septet, showing the dominance of Heisenberg exchange interaction between two Cu nuclei ( J >>A). Small out-of-plane anisotropy is observed in the spectra. In the HI spectrum the contribution ofhyperfine component is a little smaller than that in the HI,spectrum, indicating that the macrocyclehas a certain orientational order, i.e., tilted with respect to the surface normal of the film. No superhyperfine structure associated with four pyrrole nitrogens was observed due to the high concentration of the copper c0mp1ex.l~The simulations of the spectra gave the tilt angle of about 60°, a value similar to that of the PM case, between the plane normal of the majority of PD molecule and film normal. In addition, the simulated curves seem to agree better with the observed ones ifwe assume the existence of small fraction of misoriented molecules, as in the case of PM,5although (16)Wertz, J. E.;Bolton, J. R. in Electron Spin Resonance-Elementary Theory and Practical Applications; McGraw-Hill Book Co.: New York, 1972;p 250. (17)Abkowitz, M.; Monahan, A. R. J. Chem. Phys. 1973,58,2281.
Langmuir, Vol. 11, No. 10, 1995 4059
Orientation Change of Dimer-Type Porphyrin
200G
V
Figure 4. First derivative Hl ESR spectra of the LB film (40 layers on both sides) with the mixing ratio PDIC2Oln-alkane(CH~(CH&ZCH~) = 1.5/10/2.0: (a) n = 18, (b) n = 24, (c) n = 28, (d) n = 36, (e) n = 40. the detailed determination is difficult in the present case because of the weaker resolution of hyperfine structures. The spectra of the LB film containing n-hexatriacontane (the miking ratio x = 2.0) (Figure Bbyare quite different from those for the LB without n-alkane* The hyperfine component is hardly observed in the HI spectrum, and the signal is almost a singlet with unresolved hyperfine splitting. On the other hand, the septet splitting due to the All Observed in the Hll strong out-of-P1aneanisotropy Showsthat the macrocycle plane of PD is rather perpendicular to the film surface. The tilt angle of about 75” was obtained in this case. This value seems a little smaller than the case Porphyrin (80 5 0 ) , 5 which for be related to the difference of the molecular size. The spectra for the mixed LB film with x = 5.0 were similar, and those with x = 0.5 were also similar but with less pronounced anisotropy. Detailed determination of angular distribution parameters is under progress and will be described elsewhere. It should be pointed out that x = 2.0 is sufficient for the orientational change, while the area obtained from the surface pressure-area isotherms is almost constant in the range I This suggests that part Or Of n-hexatriacontane molecules in the range 0 < x I 2.0 is effective in the orientational control (mechanism 1in the preceding section),while the additional n-hexatriacontane molecules in the range 2.0 < x I 5.0 just fill the vacant sites in the LB film (mechanism 2). Similar mechanisms may also explain the PMcase wherex = O.5 was sufficient in terms of the orientation change judging from ESR spectroscopy andthe”imumofxfor thenearly constant area was 1.0a5 Effect Of the Len@h Of the Trigger Figure 4 shows the HIspectra of the mixed LB films with the mixing ratio x = 2.0 of n-alkanes with various alkyl chain lengths. In the spectrum for n-octadecane-containingfilm (Figure 4a), the shape of the spectrum is similar to the one for the LB film without n-alkane (Figure 3a), suggesting that the macrocycle plane of PD is considerably tilted with respect to the film normal; i.e., n-octadecane does not cause the orientation change of PD molecule. On the other hand, in the spectra of n-octacosane (n = 28, Figure 4c), n-hexatriacontane (n = 36, Figure 4d), and n-tetracontane
*
‘
(18)Kuroda, S.;Azumi, R.; Matsumoto, M.; King, L. G.; Crossley, M. J. Manuscript in preparation.
(n = 40, Figure 4e), theAl1 component is hardly observed, indicating that each of the macrocycle planes of PD molecules is almost perpendicular to the film surface. The spectrum for n-tetracosane (n = 24, Figure 4b) falls between the two cases. These results indicate that, when the same amount of n-alkane is introduced, there is a certain lower limit to the length of n-alkane. This lower limit seems to depend on the functional molecules being considered since n = 18 was sufficient to cause the orientational change in the PM case. This suggests that longer n-alkanes should be necessary to cause an orientational change for larger functional molecules. W N i s Absorption Spectroscopy. Polarized W / v i s spectroscopy was employed to investigate whether the PD molecule is “side-on’’or “stand-on”, (Figure 5) when its orientation is controlled by a trigger. PD has two intense absorption bands in the Soret region (424 and 454 nm).2 In the oriented liquid crystal sample the intensities of these two bands are larger when the electric vector of the incident light is parallel to the orientation direction than the perpendicular case (data not shown),indicating that the transition moments of these peaks are rather parallel to the longer axis of the m o l e ~ u l e . ~ Figure 6a shows the polarized UV/vis spectra of the mixed LB film without n-alkane at an incidence angle A YXJ *
The dichroic ratios A$A, for this LB film are 1.54 and 1.35 for the 453-nm and the 428-nm peak, respectively, suggesting that the directions ofthe transition moments of these peaks are a little different from each other. Figure 6b shows the W l v i s spectra of the mixed LB filmwithx = 2.0 ofn-hexatriacontane, The dichroicratios ofthe two bands are similar to those for the mixed LB film without n-alkane. The striking feature is that the intensity around 400 nm is larger in the p-polarized spectrum than in the s-polarized one (AAA, < 1). This was also the case for the mixed LB films containing n-alkanes with 24 and those with the mixing value = 0.5 and 5.0 ofn-hexatriacontane, onthe other hand, the dichroic ratio AJA, at around 400 nm of the LB film with where the orientation change does not judging from ESR spectra, is above l.o. These results indicate that there is a n absorption band at around 400 nm whose transition moment is rather perpendicular to the film surface when the orientation ofPD is controlled ,by a trigger, considering the nearly perpendicular orientation of the macrocycle of PD revealed by using ESR spectroscopy, the transition moment of the absorption band at 400 nm shouldbe parallel to the shorter molecular =is of PD molecule. Thus, these results will lead to a conclusion that the PD molecule is “side-on” as shown in ~i~~~ Sa, in the mixed LB films with a trigger. ~h~ role ofthe trigger molecule in the present case is such that the angle between the shorter molecular axis of PD and the film surface is controlled by the trigger (Figure 7). The smaller occupied area per PD molecule for the “side-on” model mentioned above could be explained by the vertical of PD molecules,
Conclusions This work has shown that the orientation of the functional molecule in the molecular assemblies can be controlled by a trigger molecule, n-alkane in this case. The addition of a small amount of n-alkane changes the orientation of the lightly-substituted dimer-type porphyrin, PD, in the LB films. Without n-alkane, the PD had an orientation tilted with the most probable angle 6 = 60” between the plane normal of the molecule and the film normal, while the macrocycle plane of PD was oriented more vertically (6 = 75”) with respect to the film surface
Azumi et al.
4060 Langmuir, Vol. 11, No. 10, 1995
Figure 5. Possible model of the orientation of PD molecule in the mixed LB film: (a) “side-on”and (b) “stand-on”.
/
/ /
0 00-
300
Wavelength / nm
0’301b 020a, 0 C
em 0 1 5 3
9
0.1Ot
Wavelength i nm Figure 6. Polarized UVIvis absorption spectra of the mixed LB film with the mixing ratio PD/C20/n-hexatriacontane = 1.51 10/0 (a) and 1.5/10/2.0 (b) with the 45” incidence: (solid line) p-polarized light, (dotted line) s-polarized light. for the LB film with the mixing ratio x = 2.0 of n-hexatriacontane.
Figure 7. Orientation model of PD molecule in the mixed LB film: (a) without a trigger and (b) with a trigger.
The orientation change depended on the chain length, which indicates that, as long as the amount of n-alkane is the same, there is a certain lower limit to its length for the molecule to act as a n effective trigger. It is also suggested that longer n-alkanes should be necessary to cause an orientation change for longer functional molecules. Polarized UV/vis spectroscopic studies of the n-hexatriacontane-containing LB films, combined with the study in the oriented liquid crystal, indicate that the PD molecule lies “side-on”on the film surface. The role of the trigger molecule is such that the angle between the shorter molecular axis of PD and the film surface is controlled by the trigger. Further studies are now in progress to extend this technique of controlling the orientation of functional molecules in the molecular assemblies to a series of porphyrin derivatives and other functional molecules without long alkyl chains. This will also allow us to clarify the mechanisms involved in these processes. LA950227L
